Abstract
The correction of foot and ankle deformity by means of a properly performed tendon transfer can be satisfying for both the surgeon and the patient. The goal of any tendon transfer is to create a stable, functioning, and plantigrade foot. This goal applies to every tendon transfer performed for paralysis as well, because the correction of deformity, the improvement of function, and the establishment of a plantigrade foot are essential. The goal of a tendon transfer is twofold, the first to obtain an active muscle where one does not exist, and at times the more important function is to remove the tendon as a deforming force on the foot.
Key Words
paralytic, tendon transfer, peroneal, posterior tibial, foot drop, equinovarus
Overview of Tendon Transfers
The correction of foot and ankle deformity by means of a properly performed tendon transfer can be satisfying for both the surgeon and the patient. The goal of any tendon transfer is to create a stable, functioning, and plantigrade foot. This goal applies to every tendon transfer performed for paralysis as well, because the correction of deformity, the improvement of function, and the establishment of a plantigrade foot are essential. The goal of a tendon transfer is twofold, the first to obtain an active muscle where one does not exist, and at times the more important function is to remove the tendon as a deforming force on the foot. The ability of a tendon transfer to aid in correction of deformity in non-neurologic deformity should not be underestimated. The use of an anterior tibial tendon transfer to the lateral cuneiform when correcting a varus deformity in ankle arthroplasty and peroneus brevis-to-longus transfer for flatfoot deformity are two primary examples.
Anatomy and Related Considerations
From a simplistic perspective, any muscle (tendon) that passes anterior to the ankle joint axis functions as a dorsiflexor, and conversely, any muscle or tendon passing posterior to the axis of the ankle is a plantar flexor. This is important because the peroneal tendon and the posterior tibial tendon (PTT) are plantar flexors of the foot and ankle, although their function is thought of primarily in terms of inversion and eversion. If a tendon lies centrally, in the axis of a joint, it exerts little influence on the motion of that joint. Conversely, the greater the distance a tendon lies from a joint axis, the greater force it exerts across the joint because of the longer lever arm. This is relevant, for example, in transfer of the PTT through the interosseous membrane. The transfer is performed subcutaneously, and the tendon is not passed inferior (deep) to the retinaculum, because this would decrease power. The same applies to a PTT transfer that is not inserted into the cuneiform but more proximally into the navicular. This is unfortunately sometimes necessary because one has insufficient length of the tendon to insert into the cuneiform. The same occurs with transfer of the PTT “over the top” where the tendon is passed over the medial malleolus and over the anterior leg to insert on the dorsum of the foot. However, there is not a straight line from the muscle to the insertion point as there is if the transfer is done through the interosseous membrane, and one loses about 8–10 mm of length in doing the over-the-top transfer. This is a very useful transfer, however, when the anterolateral leg is scarred and one has concerns about the viability and excursion of the transferred tendon through the interosseous membrane ( Fig. 12.1 and ).
The tibialis anterior (TA) muscle lies almost directly on top of the subtalar joint axis, but because it inserts on the medial cuneiform, it has an accessory function of inversion. At times, however, the TA can become a primary invertor of the foot—for example, in the absence of a functioning tibialis posterior muscle. The tibias anterior forms a force couple with the peroneus longus balancing the position of the first metatarsal. If there is an absent peroneus longus, particularly in childhood, the anterior tibialis will continue to power the dorsiflexion but also change the position of the first metatarsal, causing metatarsus elevatus ( Fig. 12.2A ;
The Achilles tendon lies posterior to the ankle joint axis and provides the primary plantar flexion strength for the ankle. It also normally lies slightly medial to the subtalar joint axis and therefore is a very weak invertor of the subtalar joint. This effect is negated with long-standing absence of the tibialis posterior muscle, in which case the peroneal tendons then pull the heel into valgus, potentiated by the valgus force of the Achilles tendon insertion. In patients with this deformity, the position of the Achilles tendon and thus the force of the gastrocnemius muscle have to be normalized by a medial translational osteotomy of the calcaneus, in addition to any tendon transfer performed.
The PTT and the peroneal tendons form a force couple around the ankle that controls hindfoot inversion and eversion. The PTT lies posterior to the ankle joint axis and medial to the subtalar axis. It therefore plantarflexes the ankle and inverts the hindfoot, in contrast to the peroneal tendons, which plantarflex the ankle and evert the hindfoot. Paralysis of a component of this force couple allows overpull of the antagonist, resulting in varus or valgus malalignment. Although the balance of inversion and eversion may depend on the relationship between the peroneus brevis muscle and the tibialis posterior muscle, the accessory inversion of the TA muscle and the eversion of the peroneus longus muscle (and the gastrocnemius-soleus muscle as outlined earlier) must be considered. This will determine the position of the transferred tendon, since severe varus may require a more lateral insertion of the transferred PTT ( Figs. 12.3 and 12.4 ).
In planning any tendon transfer procedure, the following factors must be considered: the relative muscle strengths and tendon excursion of every functioning muscle, no matter how weak it may appear; the positioning of the tendon to be transferred relative to the rest of the foot; the proper tensioning of a transferred tendon; and the pull-out strength necessary to secure the tendon transfer. Optimally, a tendon transfer should approximate the strength and excursion of the motor unit that it is being used to replace, but such equivalent substitution can be rarely accomplished using a single tendon. Accordingly, expecting the extensor hallucis longus (EHL) muscle to replace the TA muscle, or the flexor digitorum longus muscle to replace the tibialis posterior muscle, is unrealistic. Such a replacement can be difficult if not impossible when an attempt is made to compensate for paralysis of the strongest muscles, such as the TA or gastrocnemius-soleus, when multiple tendon transfers may be required.
Also, it is important to consider that most muscles will lose a grade of power when transferred, particularly if the transferred tendon is not phasic (a tendon that is primarily a flexor and is transferred to function as an extensor). As an example, a PTT transfer to the dorsum of the foot to regain dorsiflexion strength is not phasic, and muscle power is lost. This is unfortunately not recognized when assessing the PTT for potential transfer, and if the muscle is slightly weak, many surgeons will not use the PTT for transfer, and the deformity is corrected using a hindfoot arthrodesis, commonly a triple arthrodesis. However, the PTT inserts distal to the talonavicular joint, and if a Grade IV power posterior tibial (PT) muscle is present, this is sufficient to cause further inversion over time, causing recurrent deformity of the foot. If the PTT is transferred behind the ankle to the peroneal muscles to augment eversion, it is not functioning at a mechanical disadvantage because it has been kept posterior to the ankle axis. Use of a muscle that is phasic is always preferable because less “reeducation” of the muscle is required, rehabilitation is facilitated, and less strength of the muscle is lost in the transfer. Typically, in a PTT transfer for correction of a flaccid paralysis in which the tendon is passed through the interosseous membrane to the dorsum of the foot, at least one grade of muscle strength is lost. The same applies with another, nonphasic transfer such as use of the peroneal muscle(s) to substitute for absent ankle dorsiflexion. The peroneal muscle does not need to pass through the interosseous membrane, and these muscles can be passed more directly over the fibula to the anterior foot. Although this is a nonphasic transfer, less strength is lost than when a PTT transfer is used because the change in direction of the tendon transfer is minimized.
How tight should the transferred tendon be when secured to the bone? If the tendon is fixed to the foot at maximal elongation, the tendon transfer serves more as a tenodesis, although it always stretches out. If it is fixed in its relaxed state, however, it cannot generate adequate tension to pull effectively. In general, we prefer to insert the tendon under more tension than relaxation, because some stretching out of the muscle always occurs. A good rule of thumb is to maximally place tension on the tendon with the foot in neutral. The converse, however, does not apply, and muscle strength can never be regained if the transferred tendon is too loose. Finally, if the tendon is transferred underneath a retinaculum, which functions as a pulley, the effective tendon excursion (range of motion) is increased. With this transfer, however, the tendon is brought closer to the ankle or subtalar axes, with consequent shortening of the lever arm and reduction in strength of the transfer unit. With a subcutaneous position of a tendon transfer, excursion is decreased, but motor strength is maximized because of the greater distance from the joint axes and the resulting greater lever arm. In general, a tendon is always transferred in a subcutaneous position. Quite apart from the biomechanical advantage outlined here, the likelihood that the tendon ultimately will get “stuck,” as has been associated with transfers under the retinaculum, is greatly decreased.
Wherever possible, we perform a transfer using a tunnel with a bone-tendon-bone interference fit of the tendon. A simpler attachment of the tendon to the periosteum is never as secure. Sufficient tendon length must be present to permit its insertion in the correct location and insertion into a bone tunnel. The options for securing the tendon in the tunnel include an interference fit with an interference screw either metallic or biocomposite, and use of a suture anchor. Sometimes, we use both, which ensures excellent apposition of the tendon in the tunnel, with little tendency to pull out of the bone. The fixation of the tendon is very important, because rehabilitation with weight bearing and passive range-of-motion exercises may begin once the sutures are removed, and the strengthening and retraining that need to be initiated may start sooner. Rehabilitation is essential regardless of the type of transfer, although this is easier to accomplish if the transferred tendon is in phase with the muscle it replaced.
Timing of Procedure and Preoperative Evaluation
Recovery of muscle function may occur for up to 1 year after nerve injury. An electromyogram may have diagnostic benefit for this determination, but repeat clinical examination during this time is more helpful. Although some muscle recovery may continue up to 2 years after injury, we generally perform a transfer for paralytic deformity within 1 year after loss of function. This timing for intervention is particularly relevant when the foot is gradually deforming because of an imbalance in muscle forces about the ankle. The longer the presence of muscle imbalance, the more likely it is that fixed deformity will occur, and bone correction is required in addition to the tendon transfer. As the deformity becomes fixed, one is less able to use osteotomy for correction, and more likely to require an arthrodesis in addition to the tendon transfer. During the recovery phase after paralytic injury, the limb must be protected to prevent progressive deformity. If a protective regimen is not followed, the reconstructive procedure becomes far more difficult, if not impossible, to accomplish. A flexible equinus deformity is far easier to correct than a fixed equinovarus deformity, which may require, in addition to the tendon transfer, hindfoot and forefoot osteotomy, or arthrodesis to ensure a plantigrade foot. In the setting of a peroneal nerve injury, we have discussed the use of a combined PTT tendon transfer in addition to nerve decompression and/or grafting to maximally regain function. With this combined technique, excellent results including the ability to return to run and return to elite sport (baseball, hockey) have been achieved. Although, not everyone will achieve such a result, the important point is to understand that a fatalistic approach to a nerve injury should not be taken, and aggressive surgical reconstruction can significantly improve patient outcomes.
In evaluation of patients for possible tendon transfer, ascertaining whether the deformity is static or progressive is important. Whenever muscle imbalance is present, deformity of the foot will eventually occur, and this deterioration will be exacerbated if the muscles used for the transfer itself are involved in the paralytic process. Correction of the foot to a plantigrade position is always possible, even in a patient with a progressive deformity such as that in Charcot-Marie-Tooth (CMT) disease. If the transfer is performed in childhood, however, the initial balance of the foot subsequently may be lost if the nonfunctioning muscle then strengthens. The key to an enduring result is to create a reconstruction in which the foot is both plantigrade and balanced; even if further weakening of the muscles occurs, the foot will generally remain plantigrade. The problem that does occur is that with progressive diseases such as CMT, one may achieve the desired outcome with a plantigrade foot that has muscle function as a result of the transfer, but the muscle may weaken in the future, leading to a loss of dorsiflexion.
Fixed deformity of either the foot or the ankle cannot be corrected by tendon transfer alone, although the transfer may be integral to the success of surgery. For example, in a patient with a rigid equinovarus deformity, a triple arthrodesis may be chosen for correction. Although this procedure may initially correct the deformity, if tibialis posterior muscle function remains in the absence of peroneal strength (or vice versa in an equinovalgus deformity), deformity will recur, and a tendon transfer should be incorporated into the treatment plan ( Fig. 12.5 ). Any fixed deformity of the hindfoot must be corrected if a tendon transfer is performed. To restore passive motion across the joint on which the tendon transfer acts, the joint must be in a neutral position and the foot plantigrade. Once again, it is always preferable to use muscles that are in phase.
Tendon Transfer Procedures
Posterior Tibial Tendon Transfer
We use a four-incision technique for a PTT transfer to restore ankle dorsiflexion ( ). The operation can be done with the patient under regional or general anesthesia and positioned supine. The first incision is made medially from the level of the talonavicular joint to the medial cuneiform to harvest the PTT. The sheath is opened longitudinally, and the insertion of the tendon is exposed. An osteotome is used to remove an osteoperiosteal flap including the attachment of the PTT to the cuneiform ( Fig. 12.6A ). If possible, an additional strip of tendon with periosteum is harvested distal to the navicular. The end of the tendon is then tagged with a 2-0 suture to facilitate transfer (see Fig. 12.6B ), and the PTT sheath is split longitudinally posterior to the medial malleolus to make passage of the tendon easier (see Fig. 12.6C ).
The second incision is made medially along the calf approximately 15 cm above the level of the ankle. This corresponds to the location of the musculotendinous junction. Dissection is carried down through the subcutaneous tissue to expose the underlying fascia, which is incised longitudinally, and the tibialis posterior muscle is then palpated while pulling on the distal tendon stump (see Fig. 12.6D ). The muscle and tendon are then pulled proximally with a finger or a curved clamp. The tendon should be kept moist with a saline-soaked sponge for the remainder of the procedure. The musculotendinous junction should be maintained intact without tearing the muscle. The less fraying that occurs, the less likely it is that scarring will develop as the tendon passes laterally.
The third incision is made on the opposite side of the leg just anterior to the fibula and slightly distal to the second incision. The lateral incision must be more distal than the medial incision. When a tendon (the PTT) passes from one position to another, there must be no acute angulation of the passage of the tendon. The PTT must therefore pass in a straight line from posteromedial to distal lateral. The incision is deepened through the subcutaneous tissue, and the superficial peroneal nerve is identified and protected. The muscles of the anterior compartment are retracted medially to expose the interosseous membrane, and a 2-cm window of the interosseous membrane is excised. Care is taken to avoid injury to the underlying neurovascular bundle, which is no longer protected by the tibialis posterior muscle belly. A blunt elevator can be used to gently push aside any soft tissue structures deep to the interosseous membrane. The elevator must pass immediately on the tibia so as to avoid any neurovascular injury. Alternatively, a wide thick clamp can be used to create the window similarly passing it against the tibia from lateral to medial. The PTT is “lined up” on the lateral aspect of the leg to identify the optimal angle of passage through the interosseous space (see Fig. 12.6F ).
A large, curved clamp is then passed from the lateral aspect through the interosseous window to grab the tendon or suture medially. The clamp must be held directly against the posterior surface of the tibia, to avoid neurovascular injury. The suture is grasped, and the PTT is passed from the deep posterior compartment to the anterior compartment. Optimally, the tibialis posterior muscle belly should traverse the window in the interosseous membrane, rather than the tendon, to avoid adhesions.
The fourth incision is then made over the dorsum of the midfoot. In general, the anatomic center of rotation is the lateral cuneiform, but the appropriate point of attachment will depend on the deformity and the strength of the remaining muscles. If excessive valgus is present, then placement into the middle or even the medial cuneiform can be considered. In cases of severe varus, the cuboid is an excellent option. The soft tissues and branches of the superficial peroneal nerve are dissected and protected. The extensor tendons are then retracted and the periosteum is incised over the lateral cuneiform. A long curved clamp is passed subcutaneously from this incision to the incision over the anterolateral aspect of the leg. The suture ends are grasped, and the PTT is then passed into the incision overlying the foot. The lateral cuneiform is prepared based on the method of fixation. If a bone plug is used, then a gouge or trephine that removes the plug of bone corresponding to the diameter of the tendon ( Fig. 12.7 ) is required. The plug is removed and the tendon is advanced into the tunnel by passing the suture ends out the plantar aspect of the foot with a long straight needle. If an interference screw is chosen, and this is the preferred method of fixation, then the tendon diameter should be measured and the smallest diameter reamer that allows passage of the tendon is chosen. In many cases, the tip of the tendon is thickened and should be trimmed to facilitate passage through a smaller-diameter tunnel. The Beath pin for the reamer is assessed under fluoroscopy to ensure that it is in the center of the cuneiform, followed by use of the appropriate reamer. The suture securing the tendon is then taken with the use of the Beath pin from dorsal to plantar. If excess length of tendon is noted, a counter-incision can be made plantarly to ensure that the tendon is not “bottomed out” on the soft tissue preventing appropriate tensioning. If a suture anchor is used for fixation, it should be inserted into the cancellous bone on the side of the tunnel before the tendon is inserted. While the ankle is held in 10 degrees of dorsiflexion, the transfer is tensioned almost at maximal elongation. If necessary, a percutaneous Achilles tendon lengthening should be performed to achieve this dorsiflexion (see Fig. 12.7C ). Any lengthening of the Achilles or gastrocnemius must be done very carefully so as to avoid any further weakening of the leg muscles. A gastrocnemius recession is far safer in this regard, but clearly can only be performed if there is indeed an isolated gastrocnemius contracture present. The sutures attached to the anchor are then used to secure the tendon into the tunnel. The bone plug harvested with the gouge may then be replaced into the tunnel beside the tendon for further fixation (see Fig. 12.7B ).
